In all heaters, burners or furnaces where a flame is present, safety is always of the utmost importance. One condition which must be considered by a safety system is the absence of flame in the presence of fuel either during operation or at some time after start up. A safety control system which has been used successfully for many years employs a thermally responsive element to sense the presence of flame and a thermal time delay relay. The thermally responsive element detects a flame, or the lack thereof, while the thermal time delay relay is utilized in the safety control system when a flame is not present. In other words, a time delay is provided during start up so that sufficient time is allowed for the fuel to be ignited and heater operation to begin, or if the flame is extinguished during operation the delay provides sufficient time for re-ignition before the fuel supply is shut off.
Although such safety systems have found wide use, the American National Standards Association now recommends a maximum time in which the fuel supply must be shut off in the event that ignition does not occur or if the flame is extinguished. The thermally responsive flame sensor currently in use has a response time of approximately 30 seconds, for both heating and cooling. This response time then sets the minimum response time for the time delay relay, since it must have a longer response time to avoid unplanned shut down of the heater while attempts are being made at start up. The typical time delay for relays of this type currently in use is 45 seconds. The resulting thermal response time for such known safety systems exceeds the maximum time from flame-out to fuel valve closure which is recommended by the American National Standards Association for heaters of the flame-forced air type. Therefore, either a faster thermally responsive element must be found or a different approach adopted in providing this most important safety feature for flame heaters.